Jetdoc

The BHG phenomenon

Why does the Legend cooling system give such confusing indications when we have problems?

Air can become “trapped” in the system.
Small amount of air
If you have a tiny coolant leak a small amount of air will enter the cooling system replacing the coolant that leaked out. When enough air gets into the system, it will usually show up as an erratic idle because the coolant temp sensor is not submerged in coolant but sitting in an air gap. It will also cause you to hear coolant flowing in the heater core area, a "gurgling type sound". This sensor tells the computer to adjust the idle according to the coolant temp. This erratic idle problem usually goes away once it warms up due to the small amount of air. This sensor doesn’t work correctly in air causing the erratic idle. Bleed the air out, top off the coolant, find the small leak and repair it. Occasionally, slightly low coolant can't be explained. It just disappeared, evaporated, went to that place where missing coolant and socks go. That's why we check the coolant regularly.

Large amount of air in cooling system
If a large amount of air is introduced into the cooling system you will notice several things.
1) No heat- temp selector on hot but blowing cold air from the vents.
The heater core is full of air not hot coolant.
2) Temp gauge fluxing- The temp gauge will creep up close to hot then quickly drop back down, but not all the way back down to the normal position, but just above it.

You pull over and open the hood, you notice

3) The cooling fans are not operating but the temp needle is on hot. Most people at this point wrongly assume that the fans are causing all these problems. You cannot troubleshoot the fan system until there is no air in the cooling system. If it only overheats when stopped it may be a fan problem, if it overheats when the car is driving at normal speed it is not likely to be a fan problem.
4) The upper radiator hose is hot and swollen; the lower radiator hose is abnormally cool.

Why?
This large amount of air becomes trapped around the thermostat causing it to close, restricting the flow of coolant (this causes the hot upper hose, cool lower hose). The thermostat only works correctly when hot coolant contacts it directly. Hot air will not open it.
The fans are not working because the coolant temp sensor is also in trapped air. It is mounted close to the lower radiator hose outlet in the radiator. Cool lower hose-temp sensor cool = no fan command on.

Why the flux?
When you increase the engine rpm’s enough (+/- 3500) the water pump will eventually overcome this trapped air, pushing hot coolant to the thermostat causing it to open. The temp needle drops, the heat comes back and fans come on, but only for a short time because the cycle will repeat itself quickly.

How did the air get there?
1. A leak, if coolant leaks out air must go in. Many places for a leak. A large leak will usually result in coolant on the garage or driveway floor.
2. Cracked radiator top where throttle cable bracket attaches to radiator-ALL Legends have this problem at one time.
3. If you have no leak in the system there is only one other way to get air in the system.
INTERNAL - Head gasket leak. It could be a very small seeping leak (most common) or a large leak. The more times it gets overheated the bigger the seep/leak will get. We call it a “blown” head gasket but on the Legend it is actually a seeping head gasket.
It is always the #3 or #6 (back) cylinders. It develops a tiny seep allowing air to be pumped into the cooling system by the combustion process. At first it’s so small that only air can be pumped out of the cylinder and no coolant will be sucked into the cylinder. Like a one way check valve.
In most cases the engine will run great, there will not be any white smoke out the exhaust (until later stages) and no coolant in oil/oil in coolant, just the overheating. Some people with advanced stages of BHG will experience some white smoke.

What can I do?
If you remove the thermostat it will eliminate the “trapped” effect but the engine will not operate at the proper temperature (will be too cool). I do not recommend this in a cold climate. This will usually keep it from overheating until the head gaskets can be replaced. Some people have cut the guts out of the thermostat and installed just the outer portion only. Repeated overheating events will eventually result in warped heads and a cracked cylinder wall.

Why did this happen to me?
Most BHG’s are the result of a single overheating event (failed hose, ect.) beyond the control of the operator or continued driving with a known small leak. Do not drive the Legend with a known leak!

Many people spend months trying to figure out why there Legend is overheating so strangely. Most of us have been preprogrammed with a different background of cooling system theory. The Legend is a little different.

MY BHG STORY HERE

Jetdoc

 

Hotstartup

hmmm, even without the thermostat it will overheat

SLICK40

according to jetdoc's analysis if it is still overheating then you may have a faulty water pump. I still am not sold on the seeping HG just yet on my car, we'll see . thanks once again for the helpful post jetdoc

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Jetdoc

Hotstartup, are you saying yours still overheats with the thermostat removed?

The water pump hardly ever fails to pump. The bearings may fail and it will leak but will still pump fine.

Cole Austin

I am too tired to read this right now, but I will come back tomorrow and add any info he may have left out.

GREAT JOB!!

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RAP

You could try pumping air into the cooling system via one of those hand pumps designed for the purpose. The cooling system should not lose pressure but if it does, eliminate the radiator and cap right away. If the radiator and cap are sound then there is a leak somewhere and most likely at the headgasket or a cracked head.

Remember the radiator cap is designed to open at 16 PSI but if it fails to open you could blow your radiator open so if you use the hand pump be sure the gauge is accurate. A hairline crack in the radiator will reveal itself, at first, by leaking only when hot or under pressure. My experience was seeing drops of coolent on the fan shrouds but I couldn't find any leaks. After reading these boards I changed the radiator and all is well.

There is also a pressure tester available for the radiator cap.

Usually folks don't have these tools and go to shops but shops usually do a quick check and move on. The better way is to leave the pressure tester on for an extended period of time through a hot and cold engine cycle. If you lose pressure you have a leak and that should not be. I just went through a bad engine running hot experience ( not an Acura )
and the problem was that the overflow tank filler cap was not tight. My fault. There is no radiator cap on this car and water is added to the overflow tank, if needed.The tank is part of the pressurized cooling system and I didn't tighten the filler cap as I should have. Fortunatly no damage.

Good write up Jetdoc.

RAP

RAP

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FlipLegend

Great write up!

LPB

An excelent synopsis. I would only like to add that having the circulating coolant "wanded" using emission testing equipment will ID the smallest amount of head-gasket seepage.

There are three other sources for coolant seepage which might go unnoticed: (1) The Water (coolant) Bypass Inlet Pipe O-rings connecting the Water Passage to the Thermostat Case; (2) The O-Ring from the Coolant Connecting Pipe (under Intake Manifold) connected to the Thermostat base and the other end connected to a heater hose; and (3) The coolant hoses connected to the Fast Idle valve and the Coolant Connecting Pipe.

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Hotstartup

when my car had a bhg, i ran it w/o the thermo, it was still overheating (the needle would still go up)

Hotstartup

i dont get how it was detecting it. wierd

Jetdoc

You must have had an extreme head gasket leak. The thermostat only attempts to regulate the temperature of the coolant by restricting the flow through the cooling system. A sensor sends the temp indication to the indicator in the car. The two items are not connected.

kkiepvvessau

“blown” head gasket but on the Legend it is actually a seeping head gasket.
It is always the #3 or #6 (back) cylinders

i think it various , it not always # 3,6

Cool lower hose-temp sensor cool = no fan command on.
my fans is always on , even both lower,upper are cold. BHG Symptom

Dre Linn

i cant wait to get home and take a look at mine.

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yetchh

i didnt take the themostat out completely but drilled four 8mm holes around so the w/p could over come the bubble easily.. works really good.. wouldnt even know i had a bhg except for the fact that the rez fills up and i smell coolant when it does.. considered selling the car like that but i am sure the karma would kick me in the ass badly so changed my mind and went for the reco

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stevieray

Au contraire. 99.995% of the time the BHG occurs at #3 or #6 because they are at the farthest point in the cooling system with the exception of the heater core. I guarantee you a poll on this Forum would show the same.

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